Instant Messengers have a feature to change the current status of the active user. The different statuses range from available over do not disturb to invisible. The Instant Messenger contacts see the statuses of their buddies unless the status has been set to invisible. Invisible means that the user is online but showing as offline to his buddies.
It usually means that he can perform all actions to some of his contacts, like talking to them, while the rest thinks that he is not at his computer.
Buddy Spy is a software program for Yahoo! Messenger. It can detect if a buddy is indeed offline or invisible. The software can also check if the user is currently broadcasting with his webcam, if he is in any chat room and if he is on an alias profile or default profile.

Buddy Spy requires the creation of a new Yahoo! Messenger account, usage of the default account would interfere with Yahoo! Messenger and cause errors. After the account has been created the user can use various scans to detect if a buddy is online but invisible.
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I don’t like that at all. There’s a reason people set themselves to invisible.
Any msn live version? ;)
There are many websites to do the same.The one which I use everyday is
http://www.yahoo-status.com
Talk about taking away all of your privacy.
I use http://www.invisibleyahoo.com but also http://www.invisible-scanner.info is OK
use www.ymsgr.net .it’s 100% accurate . it’s the best
i use http://www.jdetector.com/ and never busy.it’s the best.